A phase-change random access memory (PRAM) device uses a change in an electric resistance according to a crystalline phase of a phase-change material, such as a chalcogenide material. At least a portion of a phase-change chalcogenide material layer changes into either a crystalline or an amorphous state according to a profile of an electric current passed through the chalcogenide material. The crystalline phase of the phase-change material layer, for example, can be selectively changed by altering the temperature. In other words, at least a portion of the phase-change material layer in the crystalline state may be changed by altering the temperature by controlling the profile of an electric current passed through the phase-change material layer. For example, the phase-change material layer may be transformed into the amorphous state, having a relatively high electrical resistance (i.e., a RESET state) when the phase-change material layer is rapidly cooled after applying a relatively high current pulse in a short period of time to increase the temperature of the phase-change material layer to its melting point. The phase-change material layer may be transformed into a crystalline state, having a relatively low electrical resistance (i.e., a SET state) when the phase-change material layer is cooled after applying a relatively low current pulse.
For phase-change memory devices, an important factor in decreasing energy usage during operation and increasing reliability may be reducing an amount of current used to change a state of the phase-change material layer. Accordingly, there have been attempts to increase efficiency of joule heating by reducing a contact area between the phase-change material layer and a contact plug.
A conventional phase-memory device may include a bottom electrode, a phase-change material layer, and a top electrode, in a vertical contact structure with the bottom electrode, the phase-change material layer, and the top electrode being sequentially vertically connected. (For example, refer to Stefan Lai and Tyler Lowrey “OUM-A 180 nm Nonvolatile Memory Cell Element Technology For Standard Alone and Embedded Applications, IEDM Tech Dig., 2001.) In such a structure, a current density at the two contacting areas may increase rapidly by reducing a contact area between the phase-change material layer and the electrode to thereby increase joule heating.
A conventional phase-change memory device may have a structure with a phase-change material layer extending in a plane between bottom and top electrodes. Significant heat loss may thus result.